Konqueror : Use user-defined stylesheet doesn't work

I tried : video{object-fit:none!important} in video.css ; it doesn’t work .
Is it a bug ?

Or bluntly in which file of Konqueror is its user agent stylesheet ? To edit it .

By the way I wrote this on Chrome , because on Konqueror : " Unfortunately, your browser is unsupported. Please switch to a supported browser to view rich content, log in and reply. " …

It works on Falkon ( from KDE , so why Konqueror too ? ) .

But there’s still " Unfortunately … " .